Current sheet formation and nonideal behavior at three-dimensional magnetic null points

The nature of the evolution of the magnetic field, and of current sheet formation, at three-dimensional (3D) magnetic null points is investigated. A kinematic example is presented which demonstrates that for certain evolutions of a 3D null (specifically those for which the ratios of the null point eigenvalues are time-dependent) there is no possible choice of boundary conditions which renders the evolution of the field at the null ideal. Resistive MHD simulations are described which demonstrate that such evolutions are generic. A 3D null is subjected to boundary driving by shearing motions, and it is shown that a current sheet localised at the null is formed. The qualitative and quantitative properties of the current sheet are discussed. Accompanying the sheet development is the growth of a localised parallel electric field, one of the signatures of magnetic reconnection. Finally, the relevance of the results to a recent theory of turbulent reconnection is discussed.Comment: to appear in Phys. Plasmas. A version with higher quality figures can be found at http://www.maths.dundee.ac.uk/~dpontin/ In this replacement version, typos have been corrected, and in addition references and some further discussion adde

Deadline-ordered parallel iterative matching with QoS guarantee.

by Lui Hung Ngai.Thesis (M.Phil.)--Chinese University of Hong Kong, 2000.Includes bibliographical references (leaves 56-[59]).Abstracts in English and Chinese.Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Thesis Overview --- p.3Chapter 2 --- Background & Related work --- p.4Chapter 2.1 --- Scheduling problem in ATM switch --- p.4Chapter 2.2 --- Traffic Scheduling in output-buffered switch --- p.5Chapter 2.3 --- Traffic Scheduling in Input buffered Switch --- p.16Chapter 3 --- Deadline-ordered Parallel Iterative Matching (DLPIM) --- p.22Chapter 3.1 --- Introduction --- p.22Chapter 3.2 --- Switch model --- p.23Chapter 3.3 --- Deadline-ordered Parallel Iterative Matching (DLPIM) --- p.24Chapter 3.3.1 --- Motivation --- p.24Chapter 3.3.2 --- Algorithm --- p.26Chapter 3.3.3 --- An example of DLPIM --- p.28Chapter 3.4 --- Simulation --- p.30Chapter 4 --- DLPIM with static scheduling algorithm --- p.41Chapter 4.1 --- Introduction --- p.41Chapter 4.2 --- Static scheduling algorithm --- p.42Chapter 4.3 --- DLPIM with static scheduling algorithm --- p.48Chapter 4.4 --- An example of DLPIM with static scheduling algorithm --- p.50Chapter 5 --- Conclusion --- p.54Bibliography --- p.5

Synergistic Effect of Subnanosecond Pulsed Electric Fields and Temperature on the Viability of Biological Cells

Pulsed electric fields have been used to induce a biological response in cells, and at sufficient energy, can cause cell death. By reducing the pulse duration from presently used nanosecond to subnanosecond ranges, the electric field can be delivered to biological tissue non-invasively by the use of an antenna instead of electrodes, such as needles. Studies have previously been completed in which the aim was to determine the energy density (electric field strength, number of pulses) required to induce cell death with 800 ps pulses. Based on this data, it was concluded that for pulse durations of 200 ps, with electric field strengths below 100 kV/cm, pulse numbers on the order of 106 would be needed to achieve similar effects. In this dissertation, it was shown that the energy density required for cell death can be reduced considerably if the temperature of the sample is increased to values above physiological temperature (37°C). In order to perform the experiments, a solution of biological sample (growth medium and Hepa 1-6 cells) was exposed to 200 ps pulses in which the electric field strength ranged from 60 kV/cm to 100 kV/cm and the pulse number ranged between 100 and 3,300 pulses. The temperature of the sample was controlled externally by placing the exposure chamber in a controlled temperature environment, and was varied between room temperature and 47°C. In order to reduce the thermal effects due to ohmic heating from the pulses, the repetition rate of the pulses was kept below 10 Hz. The effect, cell death, was determined by trypan blue uptake of the cells 4 hours after experimental exposure. The pulse generator used was an 8 stage Marx bank in which the output pulse was formed with a peaking and tailcut switch. The peaking switch was used to decrease the risetime of the output pulse to less than 200 ps, and the tailcut switch was used to cut off the decaying portion of the pulse. The eight spark gap switches of the Marx bank were pressurized with Nitrogen, while both the peaking and tailcut switch operated in air at atmospheric pressure. The output voltage of the pulse generator ranged from 10 kV to 20 kV and the pulse width could be varied between 140 ps and 230 ps. A conical exposure chamber was designed to expose the biological sample to the pulsed voltages, such that the electric field across the gap was homogeneous. The voltage was measured with a capacitive voltage divider, which was incorporated into the cable leading to the load. The results indicate that an increase in temperature above 37°C caused the cells to be more susceptible to the pulsed electric fields. The lethality increased to over 25% (trypan blue uptake) when the cells were exposed to 2,000 pulses with an electric field strength of 78 kV/cm at 47°C. For temperatures at and below 37°C, there was no indication of cell death, when compared to the controls, for the same pulsing conditions. In order to determine the reason for this increase in cell lethality due to the pulsed electric fields with an increase in temperature, the electrical properties of HELA 1-6 cells were measured by means of time domain reflective spectroscopy. The conductivity of the growth medium, plasma membrane, and cytoplasm increased with temperature. The permittivity of the medium and membrane increased, while the permittivity of the cytoplasm decreased with temperature. Using this data and comparing the results of the trypan blue studies, it seems to be likely that the subanosecond pulse induced cell death can be considered a dose effect with respect to the energy deposited in the cell membrane. Assuming that subcellular membranes show a similar temperature dependence in their electrical properties, the possibility that the cell lethality is triggered through permeabilization of inner membranes (in addition to that of the plasma membrane) cannot be excluded. The threshold voltage across the membrane required for electropermeabilization was shown to decrease with an increase in temperature, which likely due to the increase in the membrane fluidity with temperature. This argument is supported by molecular dynamics simulations which show an increased probability for pore formation with temperature. Based on a three layer (medium, membrane, cytoplasm) cell model and using the dielectric spectroscopy results it was concluded that the induced membrane voltage also decreases with an increase in temperature. Consequently, the threshold voltage needed to induce electropermeabilization must decrease at a faster rate than the induced membrane voltage from the electric field

Center for Aeronautics and Space Information Sciences

This report summarizes the research done during 1991/92 under the Center for Aeronautics and Space Information Science (CASIS) program. The topics covered are computer architecture, networking, and neural nets

The Predicted Binding Site and Dynamics of Peptide Inhibitors to the Methuselah GPCR from Drosophila melanogaster

Peptide inhibitors of Methuselah (Mth), a G protein-coupled receptor (GPCR), were reported that can extend the life span of Drosophila melanogaster. Mth is a class B GPCR, which is characterized by a large, N-terminal ectodomain that is often involved with ligand recognition. The crystal structure of the Mth ectodomain, which binds to the peptide inhibitors with high affinity, was previously determined. Here we report the predicted structures for RWR motif peptides in complex with the Mth ectodomain. We studied representatives of both Pro-class and Arg-class RWR motif peptides and identified ectodomain residues Asp139, Phe130, Asp127, and Asp78 as critical in ligand binding. To validate these structures, we predicted the effects of various ligand mutations on the structure and binding to Mth. The binding of five mutant peptides to Mth was characterized experimentally by surface plasmon resonance, revealing measured affinities that are consistent with predictions. The electron density map calculated from our MD structure compares well with the experimental map of a previously determined peptide/Mth crystal structure and could be useful in refining the current low-resolution data. The elucidation of the ligand binding site may be useful in analyzing likely binding sites in other class B GPCRs

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

An adaptive Cartesian embedded boundary approach for fluid simulations of two- and three-dimensional low temperature plasma filaments in complex geometries

We review a scalable two- and three-dimensional computer code for low-temperature plasma simulations in multi-material complex geometries. Our approach is based on embedded boundary (EB) finite volume discretizations of the minimal fluid-plasma model on adaptive Cartesian grids, extended to also account for charging of insulating surfaces. We discuss the spatial and temporal discretization methods, and show that the resulting overall method is second order convergent, monotone, and conservative (for smooth solutions). Weak scalability with parallel efficiencies over 70\% are demonstrated up to 8192 cores and more than one billion cells. We then demonstrate the use of adaptive mesh refinement in multiple two- and three-dimensional simulation examples at modest cores counts. The examples include two-dimensional simulations of surface streamers along insulators with surface roughness; fully three-dimensional simulations of filaments in experimentally realizable pin-plane geometries, and three-dimensional simulations of positive plasma discharges in multi-material complex geometries. The largest computational example uses up to $800$ million mesh cells with billions of unknowns on $4096$ computing cores. Our use of computer-aided design (CAD) and constructive solid geometry (CSG) combined with capabilities for parallel computing offers possibilities for performing three-dimensional transient plasma-fluid simulations, also in multi-material complex geometries at moderate pressures and comparatively large scale.Comment: 40 pages, 21 figure